Wavelength division multiplexing (WDM) enables significant increases in the data rates that can be carried over a single fiber by the use of multiple wavelengths, each carrying a separate “channel”. Time division multiplexing (TDM) techniques have limitations since the wider bandwidth required around a single base wavelength leads to impairments that limit the distance achieved. These impairments are: attenuation, reflectance, especially at splices involving flat cleaves, and chromatic dispersion due to slightly different refractive indexes at different wavelengths.
Combinations of TDM/WDM result in a capacity of 100 Gbit/s per fiber. One development has enabled the efficient application of WDM systems in real networks rather than just as point-to-point multiplex systems. The Erbium-doped fiber amplifier (EDFA) allows for the direct amplification of the optical signal without the need for intermediate electronic circuitry.
Known interrogation methods currently use fiber optic acoustic sensor arrays in what is referred to as TDM-WDM (Time division multiplexing-wavelength division multiplexing). For example, these methods currently allow for the interrogation of a greater number of hydrophones with a lesser number of laser sources.
Associated with each wavelength is a single laser source that runs continuous wave (CW). The output of the laser is gated by a fast optical switch with a low duty cycle that produces a stream of regularly spaced pulses that are amplified and sent down to a remote array of sensors dedicated to a single wavelength. For every optical pulse sent down to the sensors there are N pulses returning to the optical receiver for each of the N sensors under interrogation.
The gating process of one pulse out and N pulses returning is occurs for each laser source with its characteristic wavelength and dedicated sensors. The fast optical switch is designed to gate the light from all laser sources at the same time. This requires the multiplexing of all laser wavelengths onto the same fiber optic line with the losses associated with the multiplexing.
Each optical pulse exiting the optical gate contains all the wavelengths from the multiple laser sources. The pulse is amplified by a chain of erbium doped fiber amplifiers (EDFA's) to a peak power level that can exceed one watt. Such a high power level is required at the launch point to overcome the substantial fiber optic transmission and splitting losses experienced downstream in the fiber optic acoustic sensor system.
Various non-linear optical effects that can severely degrade over-all system performance affect light composed of multiple evenly spaced wavelengths at sufficiently high power. These effects include Brillouin scattering, Raman scattering, self-phase modulation, cross-phase modulation, and four wave mixing. Avoiding the presence of multiple wavelengths on the same fiber optic line at the same time will completely eliminate cross phase modulation and four wave mixing while mitigating the other effects.
It is desirable to use the least number of laser sources as possible to interrogate the largest number possible of sensors. For example, for each pulse of a laser source there may be 64 returning sensor pulses. However, as the number of pulses increase, so does the required bandwidth. Thus, the interrogation is limited by the available bandwidth. Therefore, there is a need for an interrogation system that is an improvement over the prior art systems.